Electrical high temperature melting furnaces of the general type shown in FIG. 1 are well known in the art as evidenced by U.S. Pat. No. 3,659,029 and the references cited therein including U.S. Pat. No. 3,580,976 and Russian Pat. No. 129,311. Generally, these furnaces utilize three electrodes A, (only one is shown in FIG. 1), spaced around an outlet member B, that also acts as an electrical conducting member cooperating with the tips of the three electrodes to form a three-pointed star current flow pattern. In most of the prior art devices it has been necessary to flood the outlet D and the electrode inlets C with an inert gas, or a reducing gas such as hydrogen, to prevent deterioration of the outlet and the electrodes, which usually are made of a refractory metal such as molybdenum, tantalum, or tungsten.
An undesirable feature of these types of prior art furnaces is that the primary electrodes were mounted such that they passed through sidewalls or the bottom of the furnace below the melt line. This electrode mounting technique requires a complex and costly furnace shell and refractory lining to accommodate the primary electrodes. Further, it requires areas that, if not insulated very precisely, can short out the electrodes and melt or warp the metal furnace shell. Also, good seals in the inlets C are required to prevent leakage of air into and around the electrodes from outside the furnace that would oxidize and shorten the electrode life, and to prevent the molten contents of the furnace from leaking out around the electrodes. As a result, and since the electrodes usually wear out before the refractory wall, changing electrodes requires interrupting the operation for substantial times to cool down the furnace, remove the damaged electrodes, and install new electrodes. Changing of the electrodes also can damage the electrical insulation between the electrodes and the shell.
Further, the optimum spacing of the electrode tips E from the outlet member B changes as the electrode tips wear which require that the electrode tips be moved in towards the center of the furnace periodically during the life of the primary electrodes. Making these adjustments has proved difficult with the prior art primary electrode mounting systems because with these systems if the primary electrodes are moved too far in towards the center they cannot be pulled back to the proper position.
While it has been known to use starting electrodes that do not pass through the furnace wall below the melt line, as evidenced by French Pat. No. 1,206,771 and U.S. Pat. No. 3,147,328, these furnaces still possess primary electrodes that pass through the furnace wall. These starting, or dipping electrodes, were used in these furnaces only during start up and were removed as soon as the material in the furnace was sufficiently melted between the primary electrode tips because these starting electrodes tended to wear very rapidly at the interface between the top of the molten glass pool and the air or unmelted batch. Thus, these electrodes were removed as soon as a sufficient current flow was obtained between the primary electrode tips.
The object of the present invention is to avoid the problems associated with bottom or below the melt line wall mounted primary electrodes by mounting specially designed electrodes in a different manner.
Another object of the invention is to provide a primary electrode arrangement that does not require flooding with a neutral or reducing gas to protect the electrodes from oxidation, and one in which the primary electrodes are easy to adjust and remove or replace without cooling down the furnace.
Still another object of the invention is to provide a primary electrode arrangement that increases the life of the electrodes as compared with the prior art and eliminates the danger of electrical shorts between the primary electrodes and the metal furnace shell.